This invention relates to flush edge protected metallic composites in the form of laminates and to a process for producing such protected laminates. Laminates protected in accordance with the process of the invention are particularly suitable as substrates for electrodes to be used in electrolytic cells and the invention is also concerned with an electrode comprising, as a substrate, an edge-protected laminate as herein described.
Bimetallic composites or laminates are known in the art and are generally produced for the purpose of utilizing the characteristics of each of the metal components to enhance the combination. For example, using a core of a light metal sandwiched between layers of a stronger but heavier metal to provide a composite which is substantially lighter than the same volume of the heavy metal alone but considerably stronger than the same volume of the lighter metal alone.
Also, it may be desirable to make a composite of a metal of high thermal or electrical conductivity with another metal of lower conductivity but greater strength to improve the strength characteristics of the composite.
For example the cladding of aluminum or copper with titanium or zirconium has been described in the journal "Metal Treatment and Drop Forging", Sept. 1954, pages 430-432.
United Kingdom Patent No. 996206 discloses a composite product having an aluminium core clad with a strongly bonded outer layer of titanium. This particular composite is produced by a method which comprises heating a core body of alumium to a temperature between 700.degree. F. and 1050.degree. F.; placing a sheet of titanium at ambient temperature in face-to-face contact with said heated core body of aluminum; and immediately passing said sheet of titanium and said aluminium core body conjointly through a rolling mill to obtain a reduction in thickness of said core body of aluminium of between 30% and 80% in one pass.
The reason for conducting the method with a cold titanium outer layer is to avoid oxidation of the titanium. However, problems may arise due to buckling or distortion of the titanium.
Such problems are overcome by the process disclosed is U.S. Pat. No. 3,711,937 issued Jan. 23, 1973 to Emley which process comprises preheating an aluminium sheet and a titanium sheet to a temperature of from about 500.degree. to 1000.degree. F., after cleaning and removing oxide from the surfaces to be bonded, bringing the cleaned, heated surfaces into momentary contact under a rolling pressure sufficient to unite the surfaces and to effect a reduction of the resultant composite sheet amounting to about 3 to 50 per cent and post-heating said composite sheet at a temperature of from about 500.degree. to 1150.degree. F. to develop the bond.
Metal composites such as those described above are useful in applications where the light weight and high electrical and thermal conductivity of the aluminum core coupled with the corrosion resistance and strength of the titanium cladding are advantageous. Thus products made from titanium-clad aluminum find their major applications in electrochemical processing, as heat exchanges and boilers, as cryogenic containers and in structural applications in the aircraft and aerospace industries.
In most of the aforesaid applications any deleterious effect from the environment, for example, excessive corrosion, on the core metal, for example aluminum, is minimal and it was not considered necessary to provide any edge protection for the metal composite. Alternatively, the composite was used in an assembly where the edges were not exposed.
However, where the environment in which the metal composite is to be used is particularly harmful or corrosive to the core metal, for example an electrolytic cell, it is highly desirable and indeed necessary that the core metal be protected, either by taking steps to avoid exposure of the edges of the composite to the corrosive environment or by providing specific edge protection where exposure of the edges to the environment is unavoidable.
An example of an environment which is corrosive to certain metals, particularly aluminum, is the electrolyte used in an electrolytic cell for the production of an alkali metal chlorate, for example, sodium chlorate. A cell for the production of sodium chlorate is disclosed in U.S. Pat. No. 3,883,406. This cell uses, as an anode, titanium coated with platinum.
U.S. Pat. No. 4,075,077 discloses an electrolytic cell having pairs of spaced perforate cathodes with flat imperforate anodes residing within each pair of cathodes. The cathodes are electrically conductive and preferably are carbon steel. The anodes are electrically conductive, preferably titanium, and are coated with a highly conductive precious metal coating. Other metals of the titanium group, i.e. zirconium, tantalum and hafnium, may be used to fabricate the anode. The precious metal coating may be platinum, a platinum iridium alloy or ruthenium oxide.
U.S. Pat. No. 4,405,418 discloses an electrolytic cell in which the anode is an electrode comprising a titanium substrate and a coating of at least one of the platinum metals or an oxide or oxygen-containing solid solution thereof. The cathode is preferably an electrode made of iron or nickel or comprised of such a metal as the substrate and a coating of nickel rhodanide or Raney nickel applied thereonto.
In each of the electrolytic cells described above the anode is preferably titanium coated with a precious metal such as platinum. While, because of the very corrosive electrolyte used in a typical sodium chlorate electrolytic cell, titanium is a suitable choice for the anode substrate, the poor electrical conductivity of solid titanium required the use of an excessively thick and expensive anode substrate. The necessary coating of a highly conductive precious metal adds to the expense.
To avoid the twin problems of poor electrical conductivity and high expense, particularly in cells requiring a high current density, it has been found that titanium clad aluminum provides an excellent anode substrate. The composite exhibits the corrosion resistance of commercially pure titanium with an electrical conductivity substantially greater than solid titanium due to the aluminum core. It is to be noted that, because of the great affinity of titanium for oxygen, to avoid build-up of non-conductive titanium oxide, it is still necessary to coat the composite with a highly conductive precious metal for efficient operation as an anode, but nevertheless a considerable economic saving is realized by using the composite.
However, use of a titamium clad aluminum composite as the anode in an electrolytic cell as described above results in another problem. The aluminum core is highly susceptible to the very corrosive electrolyte. While the upper and lower surfaces of the composite or laminate are adequately protected from corrosion by the corrosion-resistant titanium cladding, at the edges of the composite, where the core is exposed, the aluminum core is unprotected and thereby subject to rapid corrosion. Accordingly, it is necessary to protect the exposed portions of the core by providing appropriate edge protection, preferably formed from the same protective metal, e.g. titanium, as that used for the surface cladding of the laminate.
Additionally it has been found that the mere provision of a capping of protective metal which covers the exposed edge and overlaps the sides of the laminate results in inefficient operation of the cell. It is necessary that the edge protection be flush with the surface of the laminate.